9 files changed, 130 insertions, 114 deletions

diff --git a/src/objective-c/RxLibrary/GRXBufferedPipe.h b/src/objective-c/RxLibrary/GRXBufferedPipe.h
index a871ea895a..ae08cc315b 100644
--- a/src/objective-c/RxLibrary/GRXBufferedPipe.h
+++ b/src/objective-c/RxLibrary/GRXBufferedPipe.h
@@ -36,8 +36,7 @@
  * crash. If you want to react to flow control signals to prevent that, instead of using this class
  * you can implement an object that conforms to GRXWriter.
  *
- * Thread-safety:
- * The methods of an object of this class should not be called concurrently from different threads.
+ * Thread-safety: the methods of this class are thread-safe.
  */
 @interface GRXBufferedPipe : GRXWriter<GRXWriteable>
 
diff --git a/src/objective-c/RxLibrary/GRXBufferedPipe.m b/src/objective-c/RxLibrary/GRXBufferedPipe.m
index 546d46cba3..74e2f03da6 100644
--- a/src/objective-c/RxLibrary/GRXBufferedPipe.m
+++ b/src/objective-c/RxLibrary/GRXBufferedPipe.m
@@ -51,16 +51,22 @@
     // We need a copy, so that it doesn't mutate before it's written at the other end of the pipe.
     value = [value copy];
   }
-  __weak GRXBufferedPipe *weakSelf = self;
   dispatch_async(_writeQueue, ^(void) {
-    [weakSelf.writeable writeValue:value];
+    @synchronized(self) {
+      if (self->_state == GRXWriterStateFinished) {
+        return;
+      }
+      [self.writeable writeValue:value];
+    }
   });
 }
 
 - (void)writesFinishedWithError:(NSError *)errorOrNil {
-  __weak GRXBufferedPipe *weakSelf = self;
   dispatch_async(_writeQueue, ^{
-    [weakSelf finishWithError:errorOrNil];
+    if (self->_state == GRXWriterStateFinished) {
+      return;
+    }
+    [self finishWithError:errorOrNil];
   });
 }
 
@@ -100,14 +106,15 @@
 }
 
 - (void)startWithWriteable:(id<GRXWriteable>)writeable {
-  self.writeable = writeable;
-  _state = GRXWriterStateStarted;
+  @synchronized(self) {
+    self.writeable = writeable;
+    _state = GRXWriterStateStarted;
+  }
   dispatch_resume(_writeQueue);
 }
 
 - (void)finishWithError:(NSError *)errorOrNil {
   [self.writeable writesFinishedWithError:errorOrNil];
-  self.state = GRXWriterStateFinished;
 }
 
 - (void)dealloc {
diff --git a/src/objective-c/RxLibrary/GRXConcurrentWriteable.h b/src/objective-c/RxLibrary/GRXConcurrentWriteable.h
index abb831e6fb..5beca9d41e 100644
--- a/src/objective-c/RxLibrary/GRXConcurrentWriteable.h
+++ b/src/objective-c/RxLibrary/GRXConcurrentWriteable.h
@@ -23,10 +23,10 @@
 
 /**
  * This is a thread-safe wrapper over a GRXWriteable instance. It lets one enqueue calls to a
- * GRXWriteable instance for the main thread, guaranteeing that writesFinishedWithError: is the last
- * message sent to it (no matter what messages are sent to the wrapper, in what order, nor from
- * which thread). It also guarantees that, if cancelWithError: is called from the main thread (e.g.
- * by the app cancelling the writes), no further messages are sent to the writeable except
+ * GRXWriteable instance for the thread user provided, guaranteeing that writesFinishedWithError: is
+ * the last message sent to it (no matter what messages are sent to the wrapper, in what order, nor
+ * from which thread). It also guarantees that, if cancelWithError: is called (e.g. by the app
+ * cancelling the writes), no further messages are sent to the writeable except
  * writesFinishedWithError:.
  *
  * TODO(jcanizales): Let the user specify another queue for the writeable callbacks.
@@ -43,21 +43,22 @@
 - (instancetype)initWithWriteable:(id<GRXWriteable>)writeable;
 
 /**
- * Enqueues writeValue: to be sent to the writeable in the main thread.
- * The passed handler is invoked from the main thread after writeValue: returns.
+ * Enqueues writeValue: to be sent to the writeable from the designated dispatch queue.
+ * The passed handler is invoked from designated dispatch queue after writeValue: returns.
  */
 - (void)enqueueValue:(id)value completionHandler:(void (^)(void))handler;
 
 /**
- * Enqueues writesFinishedWithError:nil to be sent to the writeable in the main thread. After that
- * message is sent to the writeable, all other methods of this object are effectively noops.
+ * Enqueues writesFinishedWithError:nil to be sent to the writeable in the designated dispatch
+ * queue. After that message is sent to the writeable, all other methods of this object are
+ * effectively noops.
  */
 - (void)enqueueSuccessfulCompletion;
 
 /**
  * If the writeable has not yet received a writesFinishedWithError: message, this will enqueue one
- * to be sent to it in the main thread, and cancel all other pending messages to the writeable
- * enqueued by this object (both past and future).
+ * to be sent to it in the designated dispatch queue, and cancel all other pending messages to the
+ * writeable enqueued by this object (both past and future).
  * The error argument cannot be nil.
  */
 - (void)cancelWithError:(NSError *)error;
diff --git a/src/objective-c/RxLibrary/GRXConcurrentWriteable.m b/src/objective-c/RxLibrary/GRXConcurrentWriteable.m
index 81ccc3fbce..115195463d 100644
--- a/src/objective-c/RxLibrary/GRXConcurrentWriteable.m
+++ b/src/objective-c/RxLibrary/GRXConcurrentWriteable.m
@@ -27,8 +27,15 @@
 
 @implementation GRXConcurrentWriteable {
   dispatch_queue_t _writeableQueue;
-  // This ensures that writesFinishedWithError: is only sent once to the writeable.
+
+  // This ivar ensures that writesFinishedWithError: is only sent once to the writeable. Protected
+  // by _writeableQueue.
   BOOL _alreadyFinished;
+
+  // This ivar ensures that a cancelWithError: call prevents further values to be sent to
+  // self.writeable. It must support manipulation outside of _writeableQueue and thus needs to be
+  // protected by self lock.
+  BOOL _cancelled;
 }
 
 - (instancetype)init {
@@ -41,6 +48,8 @@
   if (self = [super init]) {
     _writeableQueue = queue;
     _writeable = writeable;
+    _alreadyFinished = NO;
+    _cancelled = NO;
   }
   return self;
 }
@@ -51,78 +60,63 @@
 
 - (void)enqueueValue:(id)value completionHandler:(void (^)(void))handler {
   dispatch_async(_writeableQueue, ^{
-    // We're racing a possible cancellation performed by another thread. To turn all already-
-    // enqueued messages into noops, cancellation nillifies the writeable property. If we get it
-    // before it's nil, we won the race.
-    id<GRXWriteable> writeable = self.writeable;
-    if (writeable) {
-      [writeable writeValue:value];
-      handler();
+    if (self->_alreadyFinished) {
+      return;
+    }
+
+    @synchronized(self) {
+      if (self->_cancelled) {
+        return;
+      }
     }
+
+    [self.writeable writeValue:value];
+    handler();
   });
 }
 
 - (void)enqueueSuccessfulCompletion {
-  __weak typeof(self) weakSelf = self;
   dispatch_async(_writeableQueue, ^{
-    typeof(self) strongSelf = weakSelf;
-    if (strongSelf) {
-      BOOL finished = NO;
-      @synchronized(strongSelf) {
-        if (!strongSelf->_alreadyFinished) {
-          strongSelf->_alreadyFinished = YES;
-        } else {
-          finished = YES;
-        }
-      }
-      if (!finished) {
-        // Cancellation is now impossible. None of the other three blocks can run concurrently with
-        // this one.
-        [strongSelf.writeable writesFinishedWithError:nil];
-        // Skip any possible message to the wrapped writeable enqueued after this one.
-        strongSelf.writeable = nil;
+    if (self->_alreadyFinished) {
+      return;
+    }
+    @synchronized(self) {
+      if (self->_cancelled) {
+        return;
       }
     }
+    [self.writeable writesFinishedWithError:nil];
+
+    // Skip any possible message to the wrapped writeable enqueued after this one.
+    self->_alreadyFinished = YES;
+    self.writeable = nil;
   });
 }
 
 - (void)cancelWithError:(NSError *)error {
-  NSAssert(error, @"For a successful completion, use enqueueSuccessfulCompletion.");
-  BOOL finished = NO;
+  NSAssert(error != nil, @"For a successful completion, use enqueueSuccessfulCompletion.");
   @synchronized(self) {
-    if (!_alreadyFinished) {
-      _alreadyFinished = YES;
-    } else {
-      finished = YES;
-    }
+    self->_cancelled = YES;
   }
-  if (!finished) {
-    // Skip any of the still-enqueued messages to the wrapped writeable. We use the atomic setter to
-    // nillify writeable because we might be running concurrently with the blocks in
-    // _writeableQueue, and assignment with ARC isn't atomic.
-    id<GRXWriteable> writeable = self.writeable;
-    self.writeable = nil;
+  dispatch_async(_writeableQueue, ^{
+    if (self->_alreadyFinished) {
+      // a cancel or a successful completion is already issued
+      return;
+    }
+    [self.writeable writesFinishedWithError:error];
 
-    dispatch_async(_writeableQueue, ^{
-      [writeable writesFinishedWithError:error];
-    });
-  }
+    // Skip any possible message to the wrapped writeable enqueued after this one.
+    self->_alreadyFinished = YES;
+    self.writeable = nil;
+  });
 }
 
 - (void)cancelSilently {
-  BOOL finished = NO;
-  @synchronized(self) {
-    if (!_alreadyFinished) {
-      _alreadyFinished = YES;
-    } else {
-      finished = YES;
+  dispatch_async(_writeableQueue, ^{
+    if (self->_alreadyFinished) {
+      return;
     }
-  }
-  if (!finished) {
-    // Skip any of the still-enqueued messages to the wrapped writeable. We use the atomic setter to
-    // nillify writeable because we might be running concurrently with the blocks in
-    // _writeableQueue, and assignment with ARC isn't atomic.
     self.writeable = nil;
-  }
+  });
 }
 @end
diff --git a/src/objective-c/RxLibrary/GRXForwardingWriter.h b/src/objective-c/RxLibrary/GRXForwardingWriter.h
index 3814ff8831..00366b6416 100644
--- a/src/objective-c/RxLibrary/GRXForwardingWriter.h
+++ b/src/objective-c/RxLibrary/GRXForwardingWriter.h
@@ -25,11 +25,7 @@
  * input writer, and for classes that represent objects with input and
  * output sequences of values, like an RPC.
  *
- * Thread-safety:
- * All messages sent to this object need to be serialized. When it is started, the writer it wraps
- * is started in the same thread. Manual state changes are propagated to the wrapped writer in the
- * same thread too. Importantly, all messages the wrapped writer sends to its writeable need to be
- * serialized with any message sent to this object.
+ * Thread-safety: the methods of this class are thread safe.
  */
 @interface GRXForwardingWriter : GRXWriter
 - (instancetype)initWithWriter:(GRXWriter *)writer NS_DESIGNATED_INITIALIZER;
diff --git a/src/objective-c/RxLibrary/GRXForwardingWriter.m b/src/objective-c/RxLibrary/GRXForwardingWriter.m
index 3e522ef24e..27ac0acdff 100644
--- a/src/objective-c/RxLibrary/GRXForwardingWriter.m
+++ b/src/objective-c/RxLibrary/GRXForwardingWriter.m
@@ -54,48 +54,65 @@
   [writeable writesFinishedWithError:errorOrNil];
 }
 
-// This is used to stop the input writer. It nillifies our reference to it
-// to release it.
-- (void)finishInput {
-  GRXWriter *writer = _writer;
-  _writer = nil;
-  writer.state = GRXWriterStateFinished;
-}
-
 #pragma mark GRXWriteable implementation
 
 - (void)writeValue:(id)value {
-  [_writeable writeValue:value];
+  @synchronized(self) {
+    [_writeable writeValue:value];
+  }
 }
 
 - (void)writesFinishedWithError:(NSError *)errorOrNil {
-  _writer = nil;
-  [self finishOutputWithError:errorOrNil];
+  @synchronized(self) {
+    _writer = nil;
+    [self finishOutputWithError:errorOrNil];
+  }
 }
 
 #pragma mark GRXWriter implementation
 
 - (GRXWriterState)state {
-  return _writer ? _writer.state : GRXWriterStateFinished;
+  GRXWriter *copiedWriter;
+  @synchronized(self) {
+    copiedWriter = _writer;
+  }
+  return copiedWriter ? copiedWriter.state : GRXWriterStateFinished;
 }
 
 - (void)setState:(GRXWriterState)state {
+  GRXWriter *copiedWriter = nil;
   if (state == GRXWriterStateFinished) {
-    _writeable = nil;
-    [self finishInput];
+    @synchronized(self) {
+      _writeable = nil;
+      copiedWriter = _writer;
+      _writer = nil;
+    }
+    copiedWriter.state = GRXWriterStateFinished;
   } else {
-    _writer.state = state;
+    @synchronized(self) {
+      copiedWriter = _writer;
+    }
+    copiedWriter.state = state;
   }
 }
 
 - (void)startWithWriteable:(id<GRXWriteable>)writeable {
-  _writeable = writeable;
-  [_writer startWithWriteable:self];
+  GRXWriter *copiedWriter = nil;
+  @synchronized(self) {
+    _writeable = writeable;
+    copiedWriter = _writer;
+  }
+  [copiedWriter startWithWriteable:self];
 }
 
 - (void)finishWithError:(NSError *)errorOrNil {
-  [self finishOutputWithError:errorOrNil];
-  [self finishInput];
+  GRXWriter *copiedWriter = nil;
+  @synchronized(self) {
+    [self finishOutputWithError:errorOrNil];
+    copiedWriter = _writer;
+    _writer = nil;
+  }
+  copiedWriter.state = GRXWriterStateFinished;
 }
 
 @end
diff --git a/src/objective-c/RxLibrary/GRXImmediateSingleWriter.h b/src/objective-c/RxLibrary/GRXImmediateSingleWriter.h
index 601abdc6b9..2fa38b3dce 100644
--- a/src/objective-c/RxLibrary/GRXImmediateSingleWriter.h
+++ b/src/objective-c/RxLibrary/GRXImmediateSingleWriter.h
@@ -23,6 +23,8 @@
 /**
  * Utility to construct GRXWriter instances from values that are immediately available when
  * required.
+ *
+ * Thread safety: the methods of this class are thread safe.
  */
 @interface GRXImmediateSingleWriter : GRXImmediateWriter
 
diff --git a/src/objective-c/RxLibrary/GRXImmediateSingleWriter.m b/src/objective-c/RxLibrary/GRXImmediateSingleWriter.m
index 3126ae4bd1..079c11b94f 100644
--- a/src/objective-c/RxLibrary/GRXImmediateSingleWriter.m
+++ b/src/objective-c/RxLibrary/GRXImmediateSingleWriter.m
@@ -20,7 +20,6 @@
 
 @implementation GRXImmediateSingleWriter {
   id _value;
-  id<GRXWriteable> _writeable;
 }
 
 @synthesize state = _state;
@@ -38,17 +37,16 @@
 }
 
 - (void)startWithWriteable:(id<GRXWriteable>)writeable {
-  _state = GRXWriterStateStarted;
-  _writeable = writeable;
-  [writeable writeValue:_value];
-  [self finish];
-}
-
-- (void)finish {
-  _state = GRXWriterStateFinished;
-  _value = nil;
-  id<GRXWriteable> writeable = _writeable;
-  _writeable = nil;
+  id copiedValue = nil;
+  @synchronized(self) {
+    if (_state != GRXWriterStateNotStarted) {
+      return;
+    }
+    copiedValue = _value;
+    _value = nil;
+    _state = GRXWriterStateFinished;
+  }
+  [writeable writeValue:copiedValue];
   [writeable writesFinishedWithError:nil];
 }
 
@@ -65,9 +63,11 @@
 // the original \a map function returns a new Writer of another type. So we
 // need to override this function here.
 - (GRXWriter *)map:(id (^)(id))map {
-  // Since _value is available when creating the object, we can simply
-  // apply the map and store the output.
-  _value = map(_value);
+  @synchronized(self) {
+    // Since _value is available when creating the object, we can simply
+    // apply the map and store the output.
+    _value = map(_value);
+  }
   return self;
 }
 
diff --git a/src/objective-c/RxLibrary/GRXWriter.h b/src/objective-c/RxLibrary/GRXWriter.h
index 5d99583a92..df4c80c28d 100644
--- a/src/objective-c/RxLibrary/GRXWriter.h
+++ b/src/objective-c/RxLibrary/GRXWriter.h
@@ -80,9 +80,9 @@ typedef NS_ENUM(NSInteger, GRXWriterState) {
  * This property can be used to query the current state of the writer, which determines how it might
  * currently use its writeable. Some state transitions can be triggered by setting this property to
  * the corresponding value, and that's useful for advanced use cases like pausing an writer. For
- * more details, see the documentation of the enum further down.
+ * more details, see the documentation of the enum further down. The property is thread safe.
  */
-@property(nonatomic) GRXWriterState state;
+@property GRXWriterState state;
 
 /**
  * Transition to the Started state, and start sending messages to the writeable (a reference to it